The present invention relates to a molding apparatus for molding resin to seal an electronic device, and more specifically to a resin plunging mechanism for the molding apparatus for plunging resin tablets into the molds in the molding apparatus.
FIGS. 1 and 2 show a molding apparatus for molding resin to seal an electronic device, for example, an IC according to U.S. Pat. No. 5,435,953. This structure of molding apparatus comprises a plurality of plungers 12a moved in and out of cavities 14a and 15a in stationary upper mold 10a and movable lower mold 11a. The plungers 12a each have a plunger head 13a. When the plungers 12a are moved into the cavities 14a and 15a in stationary upper mold 10a and movable lower mold 11a, resin tablets 16a are plunged into the molds 10a and 11a. The molding apparatus further comprises two load elements 17a driven by a motor 18a through a respective belt transmission mechanism 19a and a transmission rod 22a to move two worm gear and worm mechanisms 20a, causing the worm and worm gear mechanisms 20a to move plunger holder means 21a and the plungers 12a up and down. This plunging mechanism is complicated. Pitch error and deformation of the worm gear and worm mechanisms 20a and the belt transmission mechanism 19a affects the movement of the plunger holder means 21a, causing the plunger heads 13a unable to be synchronously moved into the cavities 14a and 15a in the molds 10a and 11a, therefore the plunger heads 13a tend to be damaged. Furthermore, during the operation of the molding apparatus, heat (molding temperature is as high as 170xc2x0 C.) is transferred from the molds 10a and 11a through plunger holder means 21a to the load elements 17a and guide bars 33a causing the load elements 17a and guide bars 33a to deform (because the worm and worm gear mechanisms 20a and guide bars 33a are securely limited on the movable plate 23a, and no horizontal gap is left between the mechanisms 20a and guide bars 33a and the movable plate 23a to absorb the space expansion of the load elements 17a and the guide bar 33a). When the load elements 17a deform, the plungers 12a are forced to tilt in different directions, and the electronic device molding quality is affected. So the plunger heads 13a are damged easily.
The present invention provides a plunging mechanism for a molding apparatus which eliminates the aforesaid drawbacks. In the plunging mechanism according to the present invention, a ball screw is rotated to move a movable base plate and two load elements at the movable base plate, enabling a plunger holder which is supported on the load elements to be moved with the movable base plate along guide bars, so that plungers which are securely mounted on the plunger holder are positively forced into cavities in upper and lower molds to plunge resin tablets.